This invention relates to expandable polyurethane powder preparations which contain blowing agents, and to their use for the production of foamed polyurethane moldings.
Powders play an important part in many areas of plastics manufacture and processing. By virtue of their high degree of dispersion and, hence, flowability, they are used, for example, in the production of adhesives, coatings or thin-films.
The production of cellular (foamed) moldings of microcellular polyurethane elastomers or polyurethane (commonly referred to as PU) foams is also known and described in numerous patent specifications and publications (cf., for example, Kunststoff-Handbuch, Vol. 7, "Polyurethan (Polyurethane)", 1st Edition 1966, 2nd Edition 1983, Carl-Hanser-Verlag, Munohen/Wien). These moldings are normally produced by introducing pourable or flowable reaction mixtures of organic polyisocyanates, compounds containing at least two reactive hydrogen atoms, and liquid blowing agents into (optionally) heated molds in which the reaction mixtures are subsequently foamed and cured.
It is known that polyurethane moldings having a compact, substantially pore-free outer skin or surface and a cellular core (i.e. integral PU foams) can be produced by introducing more of the foamable reaction mixture into the mold than would be required for completely filling the mold cavity with the foamed up mixture without the application of pressure. Foaming is carried out inside a closed mold under conditions of compression.
There are numerous applications, for example, sound-proofing in the engine compartment of motor vehicles, which require thin sheets or films of PU foam which are produced in a number of complicated individual steps by dividing up (cutting or stamping) large PU foam slabs or by bonding waste PU foam under pressure. It would be desirable to produce the foam moldings directly in the desired mold in as few operations as possible. Unfortunately, the foaming and curing of the usual liquid reactive systems in such small foam molds is difficult to carry out.
It is also known that decorative plastic films for the interior of motor vehicles can be back-foamed. The films or skins are preferably back-foamed with PU foam based on a liquid reaction mixture (cf. for example R. Pfriender, Kunststoffe, 76 (1986), 10, pages 90 et seq.). Standard practice uses thermoformed PVC/ABS films which are back-foamed in a second step which requires changing the mold. PU films produced from liquid systems may be processed by the IMC (in-mold coating) process. However, production of the components is very complicated and still not optimized (Dr. M. Wachsmann, Kunststoffberater, 10 (1987), pages 27 to 28).
By contrast, the powder slush molding process allows much finer, structured and back-foamed films (i.e. higher quality films) to be produced. The process normally uses PVC powder gelled by rotation in a mold heated to about 250.degree. C. After cooling of the mold, the film can be removed and used for back-foaming. The produced film has to be transferred to another mold, i.e. a second apparatus is needed to complete back-foaming.
One disadvantage of PVC films back-foamed with PU foams is that the PVC film and the back foaming adversely affect one another. Catalysts or stabilizers from the PU foam are capable of diffusing into the PVC surface layer. Also, plasticizers from the PVC film can migrate into the PU foam. These processes can mechanically damage the moldings, for example, by way of embrittlement or discoloration. This harmful interaction can be avoided by using a PU powder to produce the surface layers, then back-foaming with a PU foam. The resultant uniform composite is easier to further process. A simplified process in which the skin produced could be directly back-foamed in the same mold would be an advantage in this regard.
German Offenlegungsschrift 3,916,874 discloses granulating and grinding of thermoplastic polyurethanes (TPU) produced from the melt by extrusion or belt processes to obtain PU powders. These PU powders can be processed to films by sintering.
The sintering of this type of TPU powders in admixture with a blowing agent which is a solid at 23.degree. C. to form cellular PU moldings, preferably in sheet form, has also been described. These PU powder mixtures containing blowing agent may be used, for example, in the powder slush process previously mentioned to back-foam the prepared skin without having to change the mold (German Offenlegungsschrift 4.006.648).
A disadvantage of this procedure, i.e. using TPU produced by extrusion or belt processes, is the complicated subsequent grinding step required to produce the powder. Grinding must be done under intensive cooling conditions, for example with liquid nitrogen, due to the toughness of the materials used. In addition, expensive equipment is required to stop dust emissions. The irregular shape of the ground material can result in problems concerning the flowability of the powder. The high temperatures of the extrusion or belt process causes the production of the TPU powder mixture containing a blowing agent to be carried out by subsequent mixing of the powder with the blowing agent, which is also a powder solid at 23.degree. C., so an additional complicated step is involved.
It is also known that isocyanates can be reacted with monofunctional reactants ("masking agents") to form thermolabile adducts. Suitable examples of masking agents include oximes, caprolactam or phenol derivatives. Systems which are thermoplastic up to the cleavage temperature of the adducts and which crosslink beyond that temperature can be formed using these "masked isocyanates".
Uretdiones or uretdione polyisocyanates and their use in one-component reactive PU systems is also known per se. The polyisocyanates containing uretdione groups are formed by combining (i.e. "dimerizing") two isocyanate groups in the presence of special catalysts. At elevated temperatures, the uretdione groups split up to reform two isocyanate groups which continue reacting with crosslinking, for example, in the presence of OH groups. This is done without the release of a "masking agent".
In every case, thermoplastically processable systems are obtained. Upon processing, they form crosslinked structures having several advantages (e.g. higher thermal stability and tensile strengths).
the production of one-component PU system containing masked isocyanates by the conventional extrusion or belt process in the melt, which applied for thermoplastic PU, is described, for example, in the forementioned German Offenlegungsschrift 3,916,974 and in German Offenlegungsschrift 4,006,648. These systems can crosslink during production, because the decomposition temperature is reached or exceeded. In addition, the produce has to be subsequently reduced to powder, which is rather expensive.
By comparison, the direct production of PU powders known per se by polyaddition in an emulsion stabilized by interfacially active copolymers (see, for example, U.S. Pat. No. 4,985,490, and EP 0 394 789), immediately results in powders which flow freely at comparatively moderate reaction temperatures.
It was an objective of the present invention to provide suitable powder-form, fusible and, optionally, post-cross-linkable polyurethane systems for applications in which the production of cellular PU moldings with relatively small layer thicknesses or the back-foaming of compact films in the mold would be an advantage. These systems should not require an expensive grinding process or subsequent mixing with blowing agents. This objective has been met by the following process.